This invention relates to a liquid crystal display device and the manufacturing method thereof, in particular, to a liquid crystal display device having a plurality of liquid crystal layers stacked in order and the manufacturing method thereof.
The CRT (Cathode Ray Tube) display is the most popular display device used in the present time. The CRT display, however, displays all the pixels with use of one electron gun, and thus needs to have a large depth. In addition, the CRT display is so heavy and consumes so much electric power that the CRT display is not suitable as a display device of a mobile apparatus. The other display devices than the CRT display such as a plasma display device and an EL(Electro-Luminescence) display device which are now distributed in the market respectively have problems in a practical use as a mobile display device.
The liquid crystal display device is thus only one device now practically used as a display device of a mobile apparatus. The liquid crystal display device is so thin and can be driven with low electric power consumption, and thus is widely used as a display device of a mobile apparatus such as a watch and a pocket calculator. Particularly, a TN (Twisted Nematic) liquid crystal display device is incorporated with an active switching device such as a TFT (Thin Film Transistor), and thus has so good display characteristics equal to the CRT display to be used for a television. The TN liquid crystal display device, however, uses polarizers and thus has low light utilization efficiency. In order to securely obtain sufficient light to display an image, the TN liquid crystal display device needs to be provided with a backlight, which consumes a large amount of electric power.
The liquid crystal display device such as a GH (Guest Host) type liquid crystal display device using dichromatic pigment and a cholesteric selective reflection type liquid crystal display device are used as a reflective type liquid crystal display device, which needs no polarizer. In order to attain a full-color display by using the GH type liquid crystal display device, however, each sub-pixel of the display needs to be provided with liquid crystal materials different from each other in color. However, it is practically difficult to arrange such several types of liquid crystal materials in a plane. The full-color display can be also attained by stacking more than three layers of liquid crystal cells. However, when more than three layers of liquid crystal cells are stacked, various problems in a practical manufacturing process occur: the assembly of the cells, the injection of the liquid crystal material in each cells, and the mounting of the driving transistor cannot be performed with ease. The similar problems occur in using the cholesteric selective reflection type liquid crystal display device.
The full-color display can be theoretically attained by these types of the liquid crystal display devices which use no polarizer by stacking three substrates (hereinafter referred to "TFT substrates") on each of which TFTs as driving elements are mounted, and then by forming a liquid crystal layer on each TFT substrate. However, both the manufacturing cost and the weight of such a full-color display device having three TFT substrates will increase. In order to obtain a full-color display device with one TFT substrate, via conductors for connecting the liquid crystal layers on the TFT substrate each other need to be formed to reliably connect each pixel electrode and a TFT to each other.
One of the methods for attaining the full-color display by the liquid crystal device having no polarizer and only one TFT substrate has been proposed by the Jpn. Pat. KOKAI Appln. No. 6-337643. According to this method, three liquid crystal layers are stacked on one TFT substrate such that each of the liquid crystal layers holds a transparent pixel electrode between the substrate or the lower liquid crystal layers and itself, and via conductors for connecting these transparent pixel electrodes to the electrodes of the TFTs are formed by the sputtering or the printing technique in via holes which are formed in the liquid crystal layers by performing the etching thereon.
The plating technique can be also employed to form the via conductors, but the process thereof is so complicated that the number of the steps will increase. Particularly, if the etching or plating is performed after forming the liquid crystal layers, impurity ions may be introduced into the liquid crystal layers, which deteriorates the reliability of the display device. In addition, the device having the via conductors formed by the plating technique has so much connections that the failure in connection may easily occur and even disconnection may occur due to the change in temperature.
As described above, the conventional liquid crystal display device, particularly the full-color display device has so many problems in view of the manufacturing process.